The effects of the phospholipase A 2 inhibitors aristolochic acid and PGBx on A23187-stimulated mobilization of arachidonate in human neutrophils are overcome by diacylglycerol or phorbol ester

Abstract

Aristolochic acid and PGBx, two structurally unrelated, protein-targeted inhibitors of isolated phospholipases A 2, are effective antagonists of calcium ionophore A23187-stimulated mobilization of [ 3H]arachidonate from human neutrophils. We now report that preincubation of neutrophils with olcoylacetylglyccrol (OAG, 15 μM) substantially reverses the inhibitory effect of 200 μM aristolochic acid (from 70 to 24% inhibition). Similarly, OAG increases the IC 50 for PGBx from 2.5 to > 20 μM. The effects of OAG on inhibition by either aristolochic acid or PGBx are dose-dependent, with an ED 50 of 2.5 μM. Protection against inhibition by either aristolochic acid or PGBx is also observed with phorbol myristate acetate (PMA, ED 50 3 nM), but not 4-α-phorbol didecanoate. Aristolochic acid and PGBx do not inhibit PMA-stimulated superoxide generation, and are thus not protein kinase C inhibitors. Furthermore, neither aristolochic acid nor PGBx inhibit diglyceride generation through the phospholipase D/phosphatidate phosphohydrolase pathway. A23187 stimulated [ 3H]arachidonate mobilization is increased by 20–50% when neutrophils are preincubated with OAG or PMA. The present results indicate that OAG and PMA also modulate the A23187-stimulated [ 3H]arachidonate mobilization so as to render it less sensitive to inhibitors of phospholipase A 2.